1. Field of the Invention
This invention relates to a longitudinal welding method of a steel pipe, and more particularly to a welding method capable of preventing cracks (pipe end cracks) in bead ends at ends of large diameter straight seam welded steel pipes in an inner surface welding process.
2. Description of the Prior Art
A typical steel pipe longitudinally welded in the pipe manufactured by the processes that are U-ing-pressing (forming the material into U-shape in cross-section), O-ring-pressing (forming the material into O-shape in cross-section) and Expanding. The inner seam welding of a longitudinally welded steel pipe is generally effected by the submerged arc welding (SAW) or the inert gas shielded metal arc welding (MIGW). In this case, welding electrodes are supported by a boom which is guided by a preceding tracing roll moving along a groove formed in the inside of the steel pipe.
FIG. 1 exemplarily illustrates the carrying out of the inner seam welding of a steel pipe. The steel pipe 2 is so arranged on turning rolls 1 that a seam of the steel pipe 2 assumes the lowermost position. A boom 4 carrying welding electrodes 3 is inserted at one opening of the pipe into the pipe to the other opposite opening. While the boom is withdrawing from the pipe at a constant speed, the required seam welding is carried out. In the case of SAW, a device (not shown) for supplying flux 5 shown in FIG. 1 is arranged on the boom 4, while a device (not shown) for supplying a shield gas is arranged on the boom 4 for the MIGW.
In such seam weldings, there is a tendency for beads at a commencement and a termination of the welding to become defective in configuration and quality. In order to improve the quality and yield rate of the longitudinally welded steel pipes, therefore, tab plates 6 are, in general, previously welded to both ends of the steel pipe at the seam and welding is started from one point on one tab plate and terminated in on the other tab plate.
The tab plate 6 is a steel plate substantially the same in material and thickness as the steel pipe 2 and has a groove substantially the same in cross-section as the groove 7 of the steel pipe 2, thereby improving the quality and configuration of the welded bead at the ends of the steel pipe. In this case, the tab plates 6 are fixed to the steel pipe 2 by weld beads 8 by CO.sub.2 gas welding or manual welding.
In welding steel pipes, moreover, it is necessary to provide guide means for exactly moving a heat source for welding along the groove 7 to be welded because of its long weld line. It may be considered to use an optical or electromagnetic guide means which moves in advance of the heat source without touching the steel pipe. In welding the insides of the steel pipes, however, tracing rollers 9 directly in contact with the grooves 7 to trace the weld lines in advance of the heat source are usually used for this purpose in consideration of the limitation of space and security of tracing. The tracing roller 9 is directly secured to the boom 4 in a manner that all load at one end of the boom 4 more than 1 ton is transmitted to the steel pipe 2.
However, the tab plates 6 are inherently not parts of a complete steel pipe and require great material and working cost and extra cost for mounting and dismounting from the steel pipe. In order to obtain higher productivity, therefore, it is very desirable to carry out the welding without using such tab plates. Accordingly, a welding method without using the tab plates has been eagerly expected for many years, which is capable of maintaining high quality and configuration of the weld at the ends of the steel pipe.
In view of this, the inventors of this application have proposed a welding method without the above tab plates 6 and processes in connection therewith to improve the productivity, disclosed for example in Japanese Laid-open Patent Application No. 57-81,990 (Japanese Patent Application No. 156,683/80).
FIG. 2 exemplarily illustrates the method of the Japanese Laid-open Patent Application No. 57-81,990 (referred to as "prior art metal plate system", hereinafter). Namely, instead of the tab plates welded to a steel pipe, high thermal conductive metal plates 10 made of for example copper are urged against both ends of the steel pipe at the proximity of seam. Welding starts on one metal plate 10 and terminates on the other metal plate 10. If required, each of the metal plates 10 is formed with a groove 11 to put bead shapes in order. When the welding heat input is high, minimum required set plates are engaged in the grooves 11 of the metal plates before welding. Moreover, it is preferable for the high thermal conductive metal plate 10 to have a sufficient heat capacity in itself or to be positively cooled by water or the like flowing through cooling pipings 12 provided in the metal plate.
The metal plates 10 are required to have a cooling faculty for the following reasons.
If the cooling faculty of the metal plates 10 is insufficient, beads on the plates 10 or set plates themselves are angularly deformed in longitudinal section or deformed raising out of the normal shapes in a manner that the further the distance from the end of the steel pipe, the larger is the raising deformation. Such an angular deformation of the beads or set plates makes it impossible to keep the proper quality and configuration of the beads at the ends of the steel pipe. The sufficient cooling faculty of the metal plates prevents the angular deformation of the beads. This is a first reason.
Thermal stresses due to the welding heat accumulated without sufficiently dissipating in the proximity of the end of the steel pipe occur in directions shown by arrows 13 in FIG. 2, which tend to tear the weld beads. These tearing forces pull the part of the bead which has not sufficiently solidified to cause pipe end cracks so-called "hot cracking". Accordingly, it is necessary to eliminate the thermal stresses causing the tearing forces due to the welding heat or to restrain the tearing portions to suppress the tearing forces. The cooling faculty of the metal plate 10 can mainly eliminate the thermal stresses due to the welding heat. This is a second reason.
Referring back to the tab plate 6 and the weld bead 8 shown in FIG. 1 from the viewpoint aiming to prevent the deformation of the set plates and to eliminate the tearing force, the tab plate 6 should have a sufficient width in a lateral direction or in a direction perpendicular to the weld line and the weld bead 8 should have a sufficient depth of penetration with a sufficient bead length, thereby simultaneously preventing the welding heat from accumulating at the end of the steel pipe. In order to fulfil these requirements, the material, working and mounting costs of the tab plates 6 are greatly increased disadvantageously to unavoidably lower the productivity of welding processes. In view of this, the solution for the above problems has been much eagerly expected.
Under such a circumstances, the "prior art metal plate system" has been expected to remarkably increase the productivity of welding processes. With this metal plate system, although the angular deformation can be prevented, pipe end cracks frequently occur when beads are much susceptible to the hot cracking, so that this system does not sufficiently solve the problem of the pipe end cracks. Accordingly, the inventors investigated main factors for the pipe end cracks in the prior art metal plate system (FIG. 2) to find the following two factors.
(i) The pipe end cracks are caused by not only the bead tearing forces but also lateral angular deformations of the steel pipe or changing of the deformations occurring on both sides of the bead in lateral directions perpendicular to the weld line due to a load of the tracing roller. This is particularly acute in case of the metal plate system without tab plates 6.
(ii) Even if the metal plate 10 itself has an enough cooling faculty, the cooling effect only serves to cool the area of the steel pipe in contact with the metal plate 10, so that the pipe end cracks due to the tearing force cannot be sufficiently prevented because of the susceptibility of the bead to the hot cracking.
The above factors (i) and (ii) will be explained hereinafter in more detail.
First, the lateral angular deformation of the factor (i) due to the load of the (preceding) tracing roller 9 will be explained by referring to FIG. 3. In FIG. 3, a latter A illustrates a part of a steel pipe 2 as arranged on turning rollers 1 and B shows the part which has been laterally angularly deformed by weld bead 14. A letter C illustrates the part which has been further deformed by the load of the boom transmitted to the part through the tracing roller 9. In the condition B, the lateral angular deformation is caused by welding and constricting of the weld bead so as to depress the part of the steel pipe between the turning rollers 1. However, as the steel pipe is restrained by the turning rollers 1, the amount of the lateral angular deformation is relatively small in comparison with the case without the turning rollers and the weld bead is subjected to bending moments correspondingly caused by the restrained lateral angular deformation in directions tearing the weld bead during the processes of welding and contracting of the weld bead. In the condition C, on the other hand, the load of the boom is transmitted through the tracing roller 9 to the steel pipe 2 to deform the part of the steel pipe to an extent substantially equal to that in the case without the turning rollers 1, such that the weld bead 14 is not subjected to bending moments during the processes of welding and contracting of the weld bead. In the prior art using the tab plates 6 welded to the steel pipe, the hot cracking does not occur because the above condition C is maintained even when the tracing roller 9 rides on the tab plate 6.
In the process without using the tab plates as shown in FIG. 2, however, at a moment when the tracing roller 9 rides on the metal plate 10, the condition returns immediately from C to B because of no rigid connection between the metal plate 10 and the steel pipe 2, so that the bead 14 is subjected to a considerably larger bending moment than the case of variation in condition from A to B. As the result, pipe end cracks occur. In this manner, the pipe end cracks are caused by the factor other than the tearing force resulting from the thermal stresses (deformations) due to the welding heat.
Then, the factor (ii) will be explained in detail, which is the case the removal of the heat at the end of the steel pipe for reducing the tearing force is insufficient although the cooling faculty of the metal plate 10 itself is sufficient in the system without using the tab plates as shown in FIG. 2.
Even if the cooling faculty of the metal plate 10 itself is sufficient, the cooling effect serves to cool only the area of the steel pipe in contact with the metal plate and the removal of the heat is insufficient in case of the bead highly susceptible to the cracking. In the metal plate system, therefore, in addition to the removal of heat by the metal plate, the tearing force should be reduced or restrained.
As a result of a detailed investigation on the tearing force and bending moment, moreover, it has been found that in order to prevent the pipe end cracks completely, the bending moments should be reduced in addition to the decrease or restraint of the tearing force.